The present invention relates to a liquid crystal display (LCD) control system used in televisions, personal computers, work stations or the like.
In communications appliances, information appliances, audio and visual appliances, the signal processing form is recently changing from the analog signal processing to digital data signal processing. These appliances are also in the trend of downsizing, lighter weight and smaller power consumption. As the display devices used in such appliances, the conventional cathode-ray tubes (CRTs) have been gradually replaced by thin and lightweight liquid crystal panels. For example, in personal computers, work stations, televisions, visual telephones and the like, the data signals are processed digitally, and liquid crystal panels are used as display devices, and small and lightweight appliances are realized. Speaking of liquid crystal panels, in order that the liquid crystal may replace the CRT to occupy the throne of display devices, improvement of picture quality and display of high resolution must be both achieved. In the picture quality, especially, multicolor display is demanded.
For example, if eight-bit display for each of R, G, B (16,700,000 colors) which is closer to the natural color than four-bit display for each of R, G, B (4,096 colors) is realized, a color tone as close to the natural color as the present CRT display television will be obtained, and the applications of liquid crystal panel will be expanded dramatically. As for the high resolution display, if the high resolution liquid crystal panel in the number of dots as many as the high definition television (HDTV) is realized, the liquid crystal panel will play a major role in the image field in the future.
There is hence a increasing demand for a new LCD control system applicable to the multicolor and high resolution tendency, in consideration of suitability to the digital, downsizing and lower power consumption trend of appliances and the future of the liquid crystal panel.
Hitherto, in the LCD control system for driving this type of thin film transistor (TFT) liquid crystal panel, a DA converter for converting display data into analog signal and a sample hold circuit for holding and delivering the analog signal were used.
FIG. 20 is a structural diagram of a conventional LCD control system. Its operation is described below while referring to FIG. 20.
A display controller 2 delivers a gate driver control signal 4 to a gate driver 3 for controlling the gate electrode of a TFT liquid crystal panel 1. The controlled gate driver 3 renders one line valid out of arbitrary lines of the TFT liquid crystal panel 1. The display controller 2 reads out, by a display address signal 6, display data from a video random access memory (RAM) 5 for storing the digital display data.
The display data stored in the video RAM 5 is the address data of a RAM 7, and it is converted into gray scale data (color scale data in the case of a color liquid crystal panel) to be applied to the TFT liquid crystal panel 1 by the RAM 7. This is intended to curtail the memory capacity in the video RAM 5 by storing the gray scale data large in data quantity in the RAM 7 functioning as lookup table. Meanwhile, to change the display gray scale level (or display color), it is enough to change the gray scale data of the RAM 7 only, and, advantageously, it can be changed quickly.
The converted gray scale data is converted into analog signal by a DA converter 8. Conversion into analog signal is necessary because analog signal is used for charging and discharging of the display signal to the liquid crystal capacitor attached to each pixel of the TFT liquid crystal panel 1. The display signal converted into analog signal is fed into an amplifier 10 by way of an inverting/noninverting circuit 9 composed of such analog circuit as voltage follower and the like. The inverting/noninverting circuit 9 is intended to invert the polarity of the voltage applied to the liquid crystal periodically (generally in frame period), which is an essential function for driving the liquid crystal. The amplifier 10 is to boost the analog signal delivered from the DA converter 8 up to a voltage level (about 10 V) to be applied to the TFT liquid crystal panel 1. A gain and offset control circuit 11 is to adjust the gain and offset of the amplifier 10 so as to obtain the optimum display quality for liquid crystal display.
The TFT liquid crystal panel 1, unlike the CRT, must be driven in the horizontal period unit because the response speed of the TFT is slow. Accordingly, by holding the display signals for the portion of one line, they are produced simultaneously, which is called the line sequential driving.
A liquid crystal display signal 12 coming out of the amplifier 10 is fed into a sample hold circuit 15, and is sampled and held. The sample hold circuit 15 is same in the number of horizontal pixels as the TFT liquid crystal panel 1, and therefore the liquid crystal display signal 12 from the amplifier 10 is sampled sequentially by the sample hold circuit 15 designated by a shift register 13 operating at a low voltage (about 5 V).
The shift register 13 is the bits in the same number as the number of horizontal pixels of the TFT liquid crystal panel 1, and sequentially transfers pulse 18 generated from the display controller 2 in the horizontal direction by a transfer clock 17. The transfer pulse from the shift register 13 is bested (to about 10 V) by a level shifter 14, and one of the sample hold circuits 15 is selected, and the liquid crystal display signal 12 is sampled. When all display signals of one line are sampled, the display controller 2 delivers an output timing signal 16 to the sample held circuit 15, and the sample hold circuit 15 delivers an analog signal to be applied to the TFT liquid crystal panel 1. When the signal is applied to the TFT liquid crystal panel 1, one line out of arbitrary lines designated by the gate driver 3 is displayed.
In such conventional constitution of the LCD control system, however, the amplifier 10 having the gain and offset control circuit 11 is required, and in order to obtain an appropriate display quality, plural control points are needed in the gain and offset control circuit 11. Accordingly, the process required in adjustment procedure becomes a bottleneck for mass production. In the case of large-screen display, meanwhile, the data transfer speed is high, and high speed display signals pass through the DA converter 8, inverting/noninverting circuit 9 and amplifier 10, which are all analog circuits. As a result, the display signals are distorted, the display quality deteriorates, and a high resolution required for multicolor display is not obtained. Therefore, even if the DA converter 8 possesses the resolution of eight bits, the resolution after passing through the analog circuits may be lowered to about four bits to six bits.
Moreover, in the analog circuits of high speed and large amplitude, regardless of the signal level, the internal circuits possess a bias power source with a large current consumption or a reference current source, and therefore a large current is always flowing. Besides, the supply voltage of analog circuit must be sufficiently large as compared with the signal level of the analog signal to be processed in order to have a sufficient working range (dynamic range) of the operational amplifier. As a result, the analog circuits are large in power consumption, and, although the liquid crystal display device is small and light, the power supply unit becomes larger in size, and the portability is sacrificed, and the working time becomes short if working on batteries. Concerning reduction of size and weight, as compared with the digital circuit advanced in the degree of integration, the analog circuit requires a wide area for mounting to incorporate the diversified parts for composing the circuit including the resistors, capacitors and variable resistors. Thus, the conventional LCD control system involved many problems.
The invention is hence intended to solve these problems of the prior art by presenting an LCD control system reduced in the number of adjusting points, improved in mass productivity, capable of obtaining a high display quality, small in power consumption, and small in the mounting area.